Amgen has filed a federal lawsuit seeking to reverse FDA’s May 22 decision denying the company six months of pediatric exclusivity for its marketed drug Sensipar® (cinacalcet) for a new indication of secondary hyperparathyroidism (HPT) in children.

The biotech giant is accusing the FDA of violating the Administrative Procedure Act by denying pediatric exclusivity, as well as by depriving the company of its Fifth Amendment right to due process.

Amgen has named FDA Commissioner Scott Gottlieb, M.D., and Thomas Price, M.D., secretary of the U.S. Department of Health and Human Services as defendants in its lawsuit, filed May 25 in U.S. District Court for the District of Columbia. The case is being heard by U.S. District Judge Randolph D. Moss.

In its complaint, Amgen argues that as required by the Best Pharmaceuticals for Children Act, it did “fairly respond” to the agency’s Written Request that it conduct at least four studies assessing the drug in children. Amgen submitted data from those studies, and five additional studies, to the FDA in November 2016.

However, the FDA took issue with one of the four studies, Study 3, since Amgen had only 11 patients enrolled for least 12 weeks rather than the minimum 15 patients ages 28 days to 6 years old requested by the agency.

That study required at least 15 patients who would be treated for 26 weeks unless they received a kidney transplant during the study, in which case they would be treated 12 weeks before transplant. While Amgen initially enrolled 18 patients, a clinical hold triggered by the death of one patient stopped Amgen from enrolling additional patients or dosing already-enrolled patients with Sensipar.

During the hold, six patients left Study 3—four because of the hold, one because the patient received a kidney transplant, and one withdrew without being dosed. Only four patients met all study completion criteria.

“Notwithstanding these challenges, Amgen made every effort to reach the target number of completing patients in Study 3,” Amgen stated in its complaint. “The clinical hold made an almost impossible task actually impossible.”

Amgen asked the FDA to amend the Written Request to allow for the lower number of patients in Study 3, but the agency refused a meeting on the topic, the company said. Amgen said it spent between $10 million and $15 million on pediatric studies requested by the FDA.

The company has asked the court to vacate the FDA’s decision, require the agency to grant pediatric exclusivity, and award the company costs, expenses, and attorney’s fees.

Sensipar is now indicated for secondary HPT in adults with chronic kidney disease (CKD) on dialysis; hypercalcemia in adults with parathyroid carcinoma; and hypercalcemia in adults with primary HPT for whom parathyroidectomy would be indicated on the basis of serum calcium levels, but who are unable to undergo parathyroidectomy.

Pediatric exclusivity for a new indication would delay generic competition for Sensipar, a blockbuster drug that is marketed in some parts of the world as Mimpara®. Last year, Sensipar generated $1.582 billion in product sales for Amgen, up 12% from $1.415 billion in 2015. During the first quarter, the drug racked up $421 million in product sales, up 15% from $367 million in Q1 2016.